Method of achieving superior dispersions of insoluble sulfur and products thereof

ABSTRACT

A method of achieving a novel superior dispersion of insoluble sulfur comprises mixing polysulfur while it is in the rubbery state with a diluent, such as rubber, to obtain a novel uniform dispersion of the sulfur in the diluent.

The present application is a continuation-in-part of my application Ser. No. 08/187,659 filed Jan. 25, 1994, now U.S. Pat. No. 5,475,059, which is a continuation-in-part of my earlier patent application, Ser. No. 07/993,205 filed Dec. 18, 1992, now abandoned.

FIELD OF THE INVENTION

The invention relates generally to sulfur. More particularly, it relates to a novel method of achieving novel superior dispersions of insoluble sulfur in unvulcanized rubber by adding polysulfur to the rubber while the polysulfur is in the rubbery or unhardened state and to the resulting product.

BACKGROUND OF THE INVENTION

Rubber and similar polymers must be mixed with a selection of other ingredients to develop the properties necessary for specific applications. One of these ingredients is a vulcanizing agent. Elemental sulfur is by far the most widely used vulcanizing agent, especially in tires and other dynamic applications. While certain chemical compounds of sulfur can be used as sulfur donors to accomplish vulcanization, only elemental sulfur is believed to impart the optimum combination of properties for most of the tire. One of the most important properties required in tire rubber is fatigue resistance. The superior fatigue resistance achieved when using elemental sulfur instead of sulfur donors is reported in the "Natural Rubber Formulary" pp128-129, and again on pp180-181. Elemental sulfur is used in the rubber industry in two basic forms:

1. Ortho-rhombic (commonly called rhombic) crystals, consisting of molecules containing eight sulfur atoms per molecule in a ring-like structure. This form is referred to as normal, or soluble sulfur. Its main disadvantage is that it "blooms" in the unvulcanized rubber compound.

2. Polymeric sulfur (or polysulfur, as it is sometimes called to distinguish it from organic polymers containing sulfur in their polymer chains) consists of molecules that contain long chains of sulfur atoms, usually thousands of sulfur atoms per molecule. At room and processing temperatures, these chains tend to revert to normal sulfur. This reversion can be deterred by adding certain stabilizing agents in small quantities. The stabilized forms have dominated the market. Polymerized sulfur or polysulfur is referred to as insoluble sulfur. There are no known solvents for insoluble sulfur; hence its name. Its main disadvantage is that it is hard to disperse well in the unvulcanized rubber compound. It is also quite expensive compared to normal sulfur.

The degree of dispersion, in the unvulcanized rubber compound, of all of these compounding ingredients affects the properties of the vulcanized product. This is especially true of the vulcanizing agent. For the vast majority of products, such as tires, the best dispersion gives the best product because of the homogeneity achieved.

Sulfur exists at room temperatures primarily as rhombic crystals. Other forms of sulfur, such as monoclinic crystalline sulfur, or polysulfur, are the normal primary forms which elemental sulfur assumes at certain higher temperature ranges. At room temperatures, these forms convert, or revert, to rhombic sulfur.

Polysulfur is called insoluble sulfur, especially in the rubber industry, and normal, non-polymeric or rhombic sulfur is called soluble sulfur, because it is soluble to a limited extent in most rubbers. The term "rubber" as used herein means any sulfur vulcanizable polymer. Sulfur vulcanizable polymers are primarily those polymers having carbon to carbon molecular chain structures, with some double bonds existing in their structure. These polymers are called unsaturated. The double bonds are the sites for sulfur vulcanization. The term "rubbery" as used herein means masses of matter that are not hard, brittle, or friable, but are plastic and/or elastic. The term "saturated rubbery polymers" means those rubbery polymers that do not contain sulfur vulcanizable bonds, such as ethylene-propylene rubber.

The amount of normal or rhombic sulfur that is soluble in rubber increases as the temperature increases. Typical rubber compounds contain from one to three parts of sulfur per one hundred parts of rubber hydrocarbon (rhc). The processing of unvulcanized rubber requires mechanical working of the rubber, which generates heat. The temperatures developed as a result of this processing are usually sufficient to dissolve the typical normal sulfur content. When the rubber cools to room temperature the solubility of the sulfur in rubber is exceeded, and a supersaturated solution ensues. This supersaturated portion of the sulfur tends to migrate to the surface of the rubber and crystallize. This condition is called "bloom" and is highly undesirable.

At room temperatures, surface blooming occurs primarily when the concentration of soluble sulfur in the rubber is between the limits of about 0.8 parts and 8.0 parts per 100 parts of rubber hydrocarbon. These limits vary among different compounds. Below the lower limit the sulfur is soluble. Above the upper limit the sulfur drops out of solution in the interior of most rubber compounds, forming micro-crystals throughout the mix. In some rubber compounds these micro-crystals grow to objectionable size, causing nonhomogeneity of properties throughout the vulcanized product.

Polysulfur or insoluble sulfur does not dissolve in rubber, and therefore does not bloom. However, the insoluble sulfur can revert to normal sulfur, and the rate of reversion is a time-temperature phenomenon which increases with temperature. Elemental insoluble sulfur can be stabilized by the addition of various substances, notably the halogens. This stabilized insoluble sulfur tends to remain polymeric at room and processing temperatures but it reverts to normal sulfur at the higher vulcanizing temperatures, thus becoming available for the vulcanization reaction.

Insoluble sulfur is normally supplied by the sulfur manufacturers in discrete particles, or powder. This powder is extremely fine, classically having a reported average particle size of 3 microns. These particles are considerably smaller than the particles usually supplied of normal sulfur. These smaller particles are desired because the dispersion of this form of insoluble sulfur is limited by the particle size supplied, unlike the dispersion of soluble sulfur. This very fine powder presents various processing difficulties. It tends to form dust clouds in the mixing room, which are both a health hazard and a safety hazard. Sulfur dust explosions are a known hazard in the rubber industry. A number of ways to reduce this dusting are mentioned in the literature. Also, the sulfur powder is difficult to disperse in rubber. The individual particles tend to agglomerate. Because of this, the powders are frequently mixed with a portion of a polymer or other matrix materials to form a masterbatch before being added to the final compound. These masterbatches usually contain fifty percent or more sulfur. This processing step adds to the cost. Since these discrete particles retain their identity during mixing, the best possible dispersion is limited by the size of the particles, unless their melting point is exceeded. However, when melted, the rate of reversion is very rapid and the reverted sulfur, of course, blooms, and the advantages of using insoluble sulfur are negated.

The prior art falls in three categories:

1. Insoluble sulfur powders

U.S. Pat. No. 2,419,310 to Belchetz

U.S. Pat. No. 2,419,309 to Belchetz

U.S. Pat. No. 2,579,375 to Grove

These patents deal with insoluble sulfur in a form that has distinct disadvantages. The present invention overcomes these disadvantages.

2. Sulfur donors

U.S. Pat. No. 4,621,118 to Schloman

U.S. Pat. No. 2,989,513 to Hendry

U.S. Pat. No. 2,481,140 to Morris

All of these patents teach a chemical reaction of sulfur with an organic compound to form sulfur donors. The crosslinking achieved using sulfur donors is distinctly different from that achieved using elemental sulfur. No long chain polymers of sulfur are contemplated or achieved. Therefore they are not pertinent.

3. Solutions of normal sulfur

U.S. Pat. No. 1,782,693 to Miller

This patent teaches solutions of normal sulfur in an organic resin. Long chain polymers of sulfur do not go into solution in any known substance. Hence it fails to teach or suggest anything concerning polymeric sulfur.

Recently an "improved product" has been introduced, that has a reported average particle size of 2 microns. The improvement is in the degree of dispersion afforded by the smaller particle size. However, as could be expected, the dusting problem has gotten worse with the "improved product", and the dispersion is, of course, still limited by the particle size. There is still a need for much better dispersion than is achieved by the "improved product", or that can be achieved by any product that must rely on the size of a particulate to maximize its dispersion.

It is generally known that rapidly cooling molten sulfur from a sufficiently high temperature produces a mass of polymeric sulfur that is in a rubbery or unhardened state. Sulfur in this rubbery state is in a metastable condition, and upon standing, it becomes hard and brittle. It can then be ground to form a powder. In some processes the molten sulfur is sprayed and thereby cooled, the individual droplets formed in the spraying are also initially rubbery, but are then permitted to become hard and brittle. Special handling is necessary to keep them as individual particles before hardening since the rubbery state tends to make them agglomerate. In other processes, sulfur vapor is forced under pressure into a liquid cooling medium. Again rubbery particles are first formed, and special handling is required until the particles harden. The rubbery unhardened state of insoluble sulfur has been considered undesirable because it has been a free flowing powder which has been desired.

I have discovered that the metastable, unhardened, rubbery or plastic state of insoluble sulfur can be preserved by lowering the temperature of the sulfur below the glass transition temperature of the rubbery mass. Upon returning to room temperature, the mass again becomes rubbery, but upon further standing it becomes hard and brittle. When in the rubbery state this sulfur can be dispersed in rubber to obtain the superior blends or dispersions of the present invention. If desired, the sulfur can be added to the rubber mix while it is still below its glass transition temperature. When exposed to the milling temperature it quickly warms up to the rubbery state.

Other elements in Group VI_(B) of the Periodic Table of the Elements, notably Selenium and Tellurium, have many properties that are similar to sulfur. They have found limited use as vulcanizing agents for rubber, and they polymerize. It is well known that they polymerize themselves and that they form copolymers and terpolymers with sulfur. These polymers exhibit rubbery properties similar to those of the monopolymer of sulfur, and hence can be dispersed in similar fashions as the rubbery polymer of sulfur. All of these polymers are included in the term "polysulfur."

Other elements, such as Arsenic, can be included in the sulfur polymer chain, and a rubbery state can still be obtained. One skilled in the art could develop many such modifications of the sulfur polymer, and still obtain a rubbery polymer. These modifications are included in the substance of this invention. The terms "polymeric sulfur" and "polysulfur," as used herein, includes all of the modifications of the polymer that can be obtained in the rubbery state.

There is a need for ways to achieve superior dispersion of polymeric sulfur in rubber in a dust free manner. This need exists whether the sulfur is added to a rubber batch in the lower amounts needed for proper vulcanization, or in the higher amounts used in preparing masterbatches or intermediate batches, which are later added in the proportion needed to achieve the proper quantity of sulfur needed for vulcanization.

SUMMARY OF THE INVENTION

It is an object of the present invention to disclose how to achieve superior dispersions of insoluble sulfur in unvulcanized rubber, in a completely dust free manner.

It is a further object of the present invention to disclose novel superior blends made of insoluble sulfur with a diluent which is compatible with sulfur vulcanizable polymers.

It is a further object to disclose a novel dust-free method of preparing such blends.

I have discovered that a superior dispersion of insoluble sulfur is attained if polysulfur is added to the rubber while the polysulfur is in the metastable, unhardened or rubbery state. Previous to the present invention, this state has always been regarded as undesirable. The literature contains many descriptions of ways to harden this rubbery mass, so as to make it usable by prior known art.

I have further discovered that superior preparations of polymeric insoluble sulfur can be prepared by a method which comprises mixing polymeric sulfur while in the rubbery unhardened state with a compatible diluent, which is also compatible with the polymer to be vulcanized. The amount of polysulfur in the blend can vary from about 1% to about 95% by weight of the total blend. However, the preferred blends will contain about 20% to about 95% of the rubbery polysulfur by weight.

Electron Micrographs of a cross section of this superior dispersion, taken by a Leica Cambridge Ltd. S360 Scanning Electron Microscope, Version V0302A, equipped with Dynamic Stereo and Remote Control options, failed to discern any areas of unblended polysulfur. The same device clearly discerned the <1 micron to 3 micron particulates of Crystex Sulfur, the brand of insoluble sulfur that presently dominates the market because it has been the best available, in a 50/50 masterbatch dispersion of Crystex/EPR 707 (a high Mooney Viscosity Ethylene Propylene Rubber). These micrographs also showed the non-uniform dispersion of the sulfur particulates, and showed three-dimensional agglomerates of these particulates up to 29 microns across.

The blends of the present invention, since they are formed with the polysulfur in the rubbery state, eliminate the need for comminution into fine powders, thereby effecting significant economies and eliminating the hazardous formation of dust. It also eliminates the need for the rubbery sulfur polymer to harden, so that it can be processed into a powder, thereby effecting more economies. The blends of the present invention when properly prepared do not harden. They remain in the viscous or rubbery state so as to be readily dispersable by the commercial methods used to mix rubber.

In the preferred practice of the method of the present invention, the conversion process by which soluble sulfur is converted to polysulfur is interrupted when the polysulfur is in the rubbery state, and the rubbery polysulfur is mixed with the unvulcanized rubber, in the proportion needed for proper vulcanization, or with the compatible diluent, at that time.

In another embodiment, a portion of the polymer that is to be vulcanized is mixed or masticated and used as the compatible diluent for the rubbery polysulfur which is blended into it, thereby reducing the number of processing steps even further. A two roll rubber mill or other masticating equipment can be used.

In still another embodiment, normal or rhombic sulfur can be mixed in a relatively high percentage (e.g. 80%) with a compatible diluent, such as EPR (ethylene-propylene rubber, a saturated rubbery polymer) or an unsaturated rubbery polymer, and the sulfur polymerized in situ, by raising the temperature of the mixture to melt the sulfur and then rapidly cooling the mixture. This diluent is of such nature that it will not significantly react with the sulfur under the conditions of the process. This process is best done in a chambered mixer, such as an extended barrel extruder rather than on an open mill. When using the preferred high percentage of sulfur, the molten sulfur tends to separate out of the mixture and runs off the mill. In the preferred extruder, the first zone is heated and subsequent zones cooled.

In another preferred embodiment, the sulfur conversion unit is set up convenient to the intensive internal mixer, of the Banbury type. The rubbery sulfur is added to the final or intermediate batch in the quantities needed for proper vulcanization of the intended final batch. A 3 lb. capacity laboratory mixer was used.

If desired, the polymerized molten sulfur can be added directly to the rubber batch being mixed in the Banbury, using the rubber matrix as the rapid cooling medium for the sulfur.

It will be apparent to those skilled in the art that the above objects and other advantages will accompany the practice of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the preferred practice of the method of the present invention, polysulfur in the rubbery state is blended with the compatible diluent, which is rubber, by mixing the rubbery polysulfur and the diluent on a two roll rubber mill and continuing the blending or mastication until a uniform blend is obtained. The amount of the rubbery polysulfur which is used is preferably enough to produce a uniform blend which contains about 40% to about 80% of the polysulfur as based on the total weight of the blend.

The rubbery polysulfur can be prepared by any known manner. It is known that at about 113 degrees C. and above, sulfur melts. The molten sulfur begins to polymerize as the temperature is raised to about 159 degrees C. This is evidenced by a pronounced increase in viscosity. As the temperature is further increased, the viscosity increases sharply to a maximum in a rise of only a few degrees. It then drops off at a fairly steady rate as the temperature is still further increased. It is believed that the mean chain length is at its maximum at the point of maximum viscosity, and that the mean chain length decreases as the temperature rises further. The higher viscosities apparently have not been shown to inversely correlate with the percent of unpolymerized sulfur present. The purer the sulfur, the higher is the maximum viscosity. The curve of the viscosity versus temperature of pure sulfur is reversible if the temperature changes are gradual. This reversibility does not necessarily hold true when other substances, such as certain organic compounds or halogens, are present in small amounts. In pure sulfur, the end of the polymer chain is a free radical. When these other substances are present, either purposefully or accidentally, the free radical is probably "capped" by the other substances. However, for purposes of this application the term "polysulfur" is intended to cover the just-described capped polymers.

If the molten sulfur is cooled gradually from the temperature of maximum viscosity, the drop in viscosity is attributed to the reversion of the polysulfur to predominantly S₈. Other sulfur molecules, notably S₆, but others from S₂ to S₂₃ have been identified.

When the molten sulfur is cooled rapidly from temperatures whereat a significant portion of the sulfur is polymeric, the polymeric form survives to a metastable form, which is rubbery, at room temperature. In pure sulfur, the polymer reverts to predominantly S₈ in a relatively short time. This reversion is hindered by the presence of halogens or certain other substances, and seems to be accelerated by the presence of moisture, or alkalinity.

Upon rapid cooling, the molten polysulfur forms a rubbery mass of rubbery polysulfur, which hardens by itself upon standing. This hardening does not necessarily indicate de-polymerization, or reversion. Certain treatments can accelerate this hardening.

The compatible diluent which is blended with the rubbery polysulfur to form the blends of the present invention can be any substance which is compatible with both the rubbery polysulfur and the polymer which is to be vulcanized with the blend. Representative diluents include natural and synthetic rubbers, soaps, petroleum fractions, waxes, wood tar products and plasticizers, such as ethers and esters and their polymers.

The method of preparing the blends can be any of those described herein, as well as other methods which can be used to form sufficiently uniform blends.

In the following examples 1 to 6, several techniques are described which demonstrate the wide variety of sulfur conversion processes that can be used in the practice of this invention.

EXAMPLE 1

One part of bromine is added to one hundred parts of sulfur in a glass retort. Heat is applied to melt and then vaporize the sulfur. When the distillate runs in a steady stream, which must be a very thin stream, it is run into a pot of ice water which is rotating at forty-five rpm. It is collected until the stream becomes unsteady, which indicates that it will soon break. The sulfur forms a rubbery mass of strings in the ice water. It is removed from the ice water, and shook to remove most of the water. The remaining water is blown off with compressed air. The rubbery polymeric sulfur is then blended with an equal amount of natural rubber, the compatible diluent, on a two roll mill. An excellent uniform blend of the polysulfur and the rubber is obtained.

EXAMPLE 2

The procedure of Example 1 is repeated except in place of 1 part bromine, 0.8 parts of bromine and 0.2 parts of iodine are used. An excellent uniform blend of the polysulfur and the rubber is obtained.

EXAMPLE 3

The procedure of Example 2 is repeated, but the distillate of polysulfur is run directly into the nip of a cooled two roll rubber mill upon which the rubber, the compatible polymer, has already been banded and has formed a rolling nip. An excellent uniform blend of the polysulfur and the rubber is obtained.

EXAMPLE 4

One hundred parts of sulfur and 0.25 parts of bromine are melted in a beaker and heated to a point (e.g. 200 degrees C.) above the maximum viscosity of the sulfur to form a liquid. The liquid is slowly poured into a rotating ice water bath to form rubbery polysulfur which after isolation is blended with rubber, the compatible diluent, as in Example 1. An excellent uniform blend of the polysulfur and the rubber is obtained.

EXAMPLE 5

The procedure of Example 4 is repeated except that the molten liquid sulfur is fed into the compatible diluent, rubber, in the nip of the rollers, as in Example 3. An excellent uniform blend of the polysulfur and the rubber is obtained.

EXAMPLE 6

One hundred parts of sulfur and one part of iodine are melted in an aluminum dish on a hotplate and heated to spontaneous combustion. The molten sulfur is sprayed with a hot spray gun onto a cool aluminum sheet to form a thin coating of rubbery polysulfur. The coating is peeled from the aluminum sheet, and laminated with a thin sheet of broken down rubber, the compatible diluent. The laminate is rolled up and passed through the nip of a two roll rubber mill. The mixture is cross rolled several times and banded on the mill. An excellent uniform blend of the polysulfur and the rubber is obtained.

EXAMPLE 7

The uniform blends prepared as described in Examples 1-6 can be used as vulcanizing agents for sulfur vulcanizable polymers. The vulcanizing agents can be readily and uniformly dispersed in the polymer to be vulcanized. The resulting products are free of defects caused when sulfur is not uniformly dispersed in the compound.

EXAMPLE 8

    ______________________________________                                         Rubber Test Recipe                                                                        parts per                                                                      100 RHC    gms. per 3 lb. batch                                     ______________________________________                                         1. NR        50.0            420.0                                             2. SBR       50.0            420.0                                             3. HAF Carbon Black                                                                         50.0            420.0                                             4. Process Oil                                                                              3.0             25.2                                              5. Zinc Oxide                                                                               3.0             25.2                                              6. Stearic Acid                                                                             2.0             16.8                                              7. Antioxidant                                                                              1.0             8.4                                               8. Accelerator                                                                              1.0             8.4                                               9. Rubbery Sulfur                                                                           2.5             21.0                                              TOTAL        162.5    pts.   1365.0     gms.                                   ______________________________________                                    

If it is desired to initially keep selected ingredients apart, such as the sulfur and accelerator, two or more intermediate batches could be mixed separately, and later blended together, to give the desired finished batch formulation. Typical recipes for accomplishing this would be:

    ______________________________________                                                        Batch A                                                                              Batch B                                                   ______________________________________                                         1. NR            50.0    50.0                                                  2. SBR           50.0    50.0                                                  3. HAF           50.0    50.0                                                  4. Process Oil   3.0     3.0                                                   5. Zinc Oxide    3.0     3.0                                                   6. Stearic Acid  2.0     2.0                                                   7. Antioxidant   1.0     1.0                                                   8. Accelerator   2.0     0.0                                                   9. Rubbery Sulfur                                                                               0.0     5.0                                                   TOTAL            161.0   164.0                                                 ______________________________________                                    

Batch A and Batch B are each mixed. After cooling, they are blended in proportion to their batch weight to give the same finished batch as in the formula immediately above.

EXAMPLE 9

95 parts of sulfur and 5 parts of Selenium are melted together and heated to 260° C. Upon rapid cooling, a rubbery mass is formed. This is then blended with rubber. An excellent dispersion of this polysulfur, which contains Selenium, is obtained.

As previously described, the rubbery polysulfur used to make the blends may be prepared by any known method and it may or may not contain the atoms of the sulfur homologs of Group VI_(B) of the Periodic Table of the Elements, notably Selenium and Tellurium. However, it also has been found that if rubbery polysulfur is treated, either after it is formed, or as it is formed, with a solvent for soluble sulfur, such as carbon disulfide, a chlorinated hydrocarbon, an aromatic hydrocarbon, or other suitable solvent, it is possible to dissolve out some or substantially all of the soluble sulfur present. As a result, the polysulfur content of the rubbery mass can thereby be increased on a percentage basis. As a result, blends prepared from polysulfur thus treated contain less soluble or normal sulfur.

It will be apparent to those skilled in the art that a number of modifications and changes can be made without departing from the spirit and scope of the invention. Therefore, the invention is not to be limited except by the claims. 

I claim:
 1. A method of preparing a blend of a vulcanizable polymer and insoluble sulfur which comprises using as the insoluble sulfur an insoluble sulfur which is in a metastable, elastic unhardened state, said insoluble sulfur having had its metastable state preserved by lowering its temperature below its glass transition temperature.
 2. A uniform blend comprised of insoluble sulfur and a vulcanizable polymer, said insoluble sulfur having been initially dispersed while in a metastable, elastic, unhardened state and being present in an amount of about 0.8 parts to 8.0 parts per 100 parts of the vulcanizable polymer. 